The present invention relates to a sintered body of carbonitride alloy with titanium as the main component with improved properties particularly when used as the material for inserts in cutting tools for machining of metals such as turning, milling and drilling.
Sintered titanium-based carbonitride alloys, so-called cermets, are today well established as insert material in the metal cutting industry and are used especially for finishing. They contain mainly carbonitride hard constituents embedded in a binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of other composition. Their grain size is usually 1-2 .mu.m.
In addition to Ti, other metals of the groups IVA, VA Hf, V, Nb, Ta, Cr, Mo and/or W, are normally found in the carbonitride hard constituents but may also be present as carbide and/or nitride hard constituents. The binder phase generally contains cobalt as well as nickel. The amount of binder phase is generally 3-30% by weight.
It is known that different kinds of core-rim structures can be created by adding different alloying elements to a titanium-based carbonitride alloy. By changing the core-rim structure, it is possible, e.g., to change the wettability in order to facilitate sintering. It is also possible to change the properties of the sintered body, for example, to increase the toughness or resistance against plastic deformation as disclosed in, e.g., U.S. Pat. Nos. 3,971,656 and 4,857,108 and Swedish Application No. 8902306-3.
The positive effects of the rim phase stated above has to be balanced with the fact that the rim phase is as brittle but not as hard as the core phase. This is believed to result in crack propagation being concentrated to the rims.
The rims are formed during sintering. The amount of rim that grows on a core is dependent on the sintering temperature and on the chemical composition of the alloy and the core. It is generally believed that the amount of rim formed on a core decreases with increasing amount of nitrogen in the alloy. For alloys with N/(C+N)&gt;0.5, hardly any rims at all are found.
U.S. Pat. No. 4,957,548 discloses a titanium-based carbonitride alloy containing 50% by volume or less particles of TiN or TiCN with N.gtoreq.C with no core-rim structure. The starting materials are milled in the conventional way and, thus, have an angular grain morphology.
During liquid phase sintering, grain growth is driven by an Ostwall ripening process. For WC-Co alloys, the grain growth of the WC is highly orientated. This orientated growth also exists in titanium-based carbonitride alloys. It is mainly the rims on Ti-containing cores that exhibits this growth orientation. This is evident from the micrograph, FIG. 1, where angular Ti containing cores can be seen. The core-rim interface is straight lined/plane and the interfaces are orientated to certain low energetic crystallographic planes. On top of these cores, rims have grown on the straight lined interface. The interfaces between these rims and the binder phase are also angular and have a low energetic interface plane. All this is even better shown in the TEM micrographs (FIGS. 2 and 3).